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search Group, POSCO Technical Research Laboratories 699, Gumho-dong, Gwangyang-si, Jeonnam, 545-090, Korea mkpark98@posco.comAbstract. The application of the hot press forming part is extended father because it has good formability at high temperature, high strength after rapid cooling in the dies and low springback. Major technologies to develop hot press forming tools are die face design and die cooling system design. In this paper, the forming simulation using coupled thermo-mechanical analysis for dice face design and flow, heat transfer simulation for die cooling system design of Bpillar are performed. Mass production tools of B-pillar are made and dimensional accuracy and hardness of part are checked. And the forming, flow and heat transfer simulation results of hot press formed part are compared with the experimental ones to confirm the validity of the purposed simulations. Keywords: Hot press forming, Forming simulation, Flow simulation, Heat transfer simulation PACS: 02.60.Cb

INTRODUCTIONIn recent years advanced-high and ultra-high strength steels are increasingly used in the automotive industry because of weight reduction and improvement in crashworthiness. When forming advanced-high and ultra-high strength steels, an increase in strength usually leads to some problems such as reduced formability and tendency to springback. To produce high strength steel part for light weight vehicle, hot press forming(named as hot stamping in other literatures) has been developed and applied. During hot press forming process blank was heated up and formed at high temperature, complex shaped part can be produced. The part produced by hot press forming has much lower springback than other high strength steel forming process due to low flow stress at high temperature and rapid cooling in dies. For the rapid development of hot press formed components, it is good to use forming, refrigerant flow and heat transfer simulations. The prototype manufacturing and testing of tools which cost much expense and time can be avoided by using finite element simulations on the manufacturing process. A finite element simulation of the forming process and rapid cooling in dies should consider the thermal-mechanical contact between the tool and the sheet metal to obtain accurate predictions of temperature distribution, cooling rate, stresses and strains. Several researchers have studied simulation for hot press forming and hot press forming die design. Altan [1] reviewed the forming and cooling simulation for hot press forming. According to Altan [1], an insufficient cooling rate because of poor tool design can be predicted and corrected using finite element simulation. Besides temperature distribution, thermal finite element simulation also can be used to predict the cycle time required to cool a part to a certain temperature when a part can be safely withdrawn from the die. Thus, a controlled and optimized hot process foring process can be developed with finite element simulations [2]. And some researchers have studied coupled thermo-mechanical simulation on warm and hot forming processes of metal sheets. Yoshihara et al. [3], Palaniswamy et al. [4], and Takuda et al. [5] carried out finite element simulation for warm forming process of magnesium or aluminium alloy sheet. In this paper, hot press forming B-pillar die was developed based on forming, refrigerant flow and cooling simulations. For die face design, forming simulation was done using high temperature properties of POSCO boron steel. For cooling CREDIT design in dies, TO BE INSERTED ON THE FIRST PAGE OF EACH PAPER in cooling holes and system LINE (BELOW) we carried out flow simulation to estimate water flowEXCEPT THE PAPER ON PP. 837 TO 840

heat transfer simulation to estimate part ejecting temperature during continuous operation. Some simulation results were compared with experimental data.

COUPLED THERMO-MECHANICAL FORMING SIMULATIONThe blank in the hot press forming process is usually formed in a temperature range between 600℃ and 800℃. For a finite element forming simulation, thermo-mechanical material characteristics of the hot press forming process are required. For investigation of the material’s flow stress in dependency of the temperature and the strain rate between 630°C and 780°C in a range of strain rate (0.002/s to 1/s) by austenitic state hot uniaxial tensile tests. The detailed mehtod and results are described in our other paper[6]. Son[6] investigated the influence of temperatures and strain rates on the flow properties of the test material and found that the temperature has a significant impact on the forming behavior of a boron steel. Increasing the temperature leads to a significant reduction of the flow stress and a decreasing work hardening exponent, resulting in a remarkable decrease of the slope of the true stress-strain curves. It was also found that the strain rate has a significant influence on the forming behavior of the boron steel. Increasing the strain rate leads to appreciable increase of the stress level and the slope of the curve. To use the flow curves in the simulation by LS-DYNA, the flow curves were expressed as a Cowper-Symonds model using experimental data and assumed by extrapolation at other strain rates. B-pillar forming process was simulated with coupled thermo-mechanical forming simulation and produced with mass tool as shown in Fig. 1. LS-DYNA, the explicit dynamic FEM code, was used for forming simulation. In forming simulation, shell elements were used for tools and a blank and the element size of a blank was 2mm. Thermal and mechanical contacts were defined using ‘contact_surface_to_surface_thermal’ card. In hot press forming process, a blank of 1.7 mm in thickness cold rolled sheet was heated to 900℃ and held at that temperature for 5 min and then placed on the binder in 12sec. The forming time was about 2sec. At the upper die moved down to contact to the blank, the temperature of a blank was about 750℃ and the blank was rapid cooled in the dies for about 20 seconds, after an upper die reached at the bottom position.The thinning distribution and the strain distribution on a FLD at the final state of forming simulation are shown in Fig. 2. It is seen that maximum thinning happened in a region A and maximum thickening happened in a region B. Simulation results well predicted experimental results as shown in parenthesis for thinning at several positions of a hot press formed B-pillar. And we check part shape precision over 50 points. Coordinate measurement machine was used to check shape precision. Almost every shape presicion check point of part meet the standard before shot blast process.

FIGURE 1. Tool set-up position and 3D mass tool model of B-pillar

FIGURE 2. Thinning and strain distribution at the final stage

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FLOW AND HEAT TRANSFER SIMULATIONIn hot press forming process, the blank is formed in a temperature range between 600℃ and 800℃ and rapid cooled in dies to achieve high strength. To secure rapid cooling rate enough for full martensite transformation of the forming part during continuous operation, the hot press forming die should be cooled rapidly and efficiently. The die must absorb and dissipate the heat energy from the heated blank and generally cooling holes or channels are machinned at hot press forming dies. Cooling holes or channels are designed to achieve a uniformly distribution of mechanical properties in the formed part. Generally the dies for hot press forming part have many cooling holes and cooling hole system is very complicate because of the part shape variation and the interference of cooling hole, ejector for automation or blank guide pin. When the cooling holes are located near from the die face and other cooling hole, cooling efficiency is better than that with cooling hole far from the die face and other cooling hole. In this paper, die cooling system was designed with 3D modeling tool and flow and heat transfer simulations. Refrigerant flow simulation was performed with FLUENT and heat transfer simulation was performed with LS-DYNA. Figure 3 shows the cooling hole system of B-pillar upper and lower tool. To get uniform cooling efficiency and uniform distribution of mechanical properties in the formed part, we set a limit on the maximum number of cooling hole divergency and layer. When modeling the B-pillar cooling hole system, the maximum number of divergence is 4 and maximum number of layer is 3. Figure 4 shows flow and heat transfer simulation results for refrigerant flow and temperation distribution of the tool at the time of 20 seconds after upper die reached at the bottom position during continuous operation. Part ejecting temperature of hot press forming part under serial production was also shown in Fig. 4. The inlet flow rate is about 60 liter per minute. Flow simulation is used to determine the heat convection coefficient of cooling hole surface for heat transfer simulation. The interface heat transfer coefficient and thermal properties for tool and blank are obtained from the literature.

FIGURE 3. The cooling hole system of B-pillar upper and lower tool

FIGURE 4. The results of flow and heat transfer simulation and part ejecting temperation under serial production

Figure 5 and Table 1 show experimental data for Rockwell hardness distribution at the surface of the hot press formed part. It is seen that the hardness is over than 45HrC (TS≈1500MPa) at every cheking point as expected from the heat transfer simulation. According to the simulation result, the cooling rate of the blank was over than 30°C/sec, thus very high strength can be expected from CCT curve of the boron steel.

CONCLUSIONIn this paper, hot press forming B-pillar mass tool was designed by using forming, refrigerant flow and heat transfer simulations. High temperature properties such as flow curves considering strain rate and temperature were used for forming simulation. When modeling the cooling hole system, the maximum number of divergence is 4 and maximum number of layer is 3. Flow and heat transfer simulation were used for evaluate the efficiency of cooling system and estimate part ejecting temperature during continuous operation. Some simulation results were compared with real part.